This invention relates to a circuit for triggering optical instrumentation and more particularly to a trigger for a ballistic optical camera used in the art of photographing projectiles, such as bullets, wherein the ballistic optical camera trigger has at least one circuit component along the flight path of a projectile for detecting passage of the projectile in order to trigger a camera to take an image of the projectile no matter what the velocity or change in velocity of the projectile.
An existing trigger system known as a "ZERO TIME RADAR" provided by Terma used radar technology similar to the radar used to trigger x-ray units and zero time for ballistic measurements, wherein such similar radar would cause a false triggering of a camera. Further, x-ray "splatter" would also cause a false triggering of the camera down the line of flight of the projectile.
A further triggering method uses an Infra Red (shotgun) trigger which triggers on the flash at the end of a muzzle and delay has to be calculated for intercepting the projectile at the proper point of the camera. Such IR triggers do not work well since obturation of rounds of munitions vary from round to round and creates a very undesirable jitter of the image in the field of view of the camera or loss of data.
Both the "ZERO TIME RADAR" and the IR trigger are analog in nature and light, heat, and RF noise cause false triggers and lost data.
In U.S. Pat. Nos. 4,128,761 and 4,845,690, Kenneth L. Oehler describes ballistic measurement devices known as chronographs wherein a projectile is detected by at least a pair of optical sensors, i.e. "screens" or "skyscreens", for starting and stopping a device for measuring the time the projectile passed from a first of the optical sensors to the second of the optical sensors. Using this measurement and the known distance between the two optical sensors, the velocity of the projectile, at a certain point in the projectiles flight path, can be determined. Errors occurring in these types of devices with these types of optical sensors are generally caused by a faulty sensor. In order to overcome such errors another pair of optical sensors, independent of the first pair, is provided to provide verification of operation of the first pair. However, in the U.S. Pat. No. 4,845,690 device Oehler describes the use of three sensors instead of four to compensate for error due to a faulty one of the sensors. These types of chronograph systems require at least two optical sensors in order to operate properly, and the distance between the sensors must be predetermined.
The Synchronizing Circuit described by Barry J. Baxter in U.S. Pat. No. 4,418,999 utilizes two spaced apart magnetic sensors, each of which start a corresponding counter to start counting in a predetermined direction when a projectile is detected by each sensor in order to output a trigger pulse to operate a flash for a camera at a precise point downstream from the sensors and along the flight path of the projectile, irrespective of the speed of the projectile. This circuit, like the chronograph, requires the use of at least two sensors, and the distance from the sensors to the camera must also be known. Accordingly, this circuit involves a delay in activating the flash of the camera due to computational delays.